There is evidence that fibronectin is involved in many biological processes including tissue repair, embryogenesis, blood clotting, cell migration, wound repair, and cell adhesion. There are two primary forms of fibronectin. The first is an insoluble glycoprotein dimer that serves as a linker in the extracellular matrix (ECM), and the second is a soluble disulfide-linked dimer found in plasma. The ECM form of fibronectin is expressed by fibroblasts, chondrocytes, endothelial cells, macrophages and certain epithelial cells. The plasma form of fibronectin is expressed by hepatocytes. Fibronectin can serve as a general cell adhesion molecule by anchoring cells to collagen or to proteoglycan substrates. Fibronectin can also play a role in organizing cellular interactions by binding to components of the ECM and to membrane-bound fibronectin receptors on cell surfaces. Forms of fibronectin are found in vertebrates, including mammals, birds, amphibians, fish, and reptiles.